Organic solution of dialkyl peroxydicarbonate, process for producing it, preparation of halogenated polymers with the involvement of this solution and halogenated polymers obtained

ABSTRACT

The invention relates to a process for the preparation of an organic solution of dialkyl peroxydicarbonate which is particularly suited for use in the aqueous suspension polymerization of halogenated monomers. The dialkyl peroxydicarbonates are employed in the aqueous suspension polymerization of halogenated monomers in the form of an organic solution in a water-insoluble organic solvent chosen from conventional chain-regulating agents for halogenated polymers.

The present invention relates to organic solutions of dialkylperoxydicarbonate and to their use in the polymerization of halogenatedmonomers for the purpose of obtaining polymers with improved properties.

It is known to resort to dialkyl peroxydicarbonates in order to initiatethe aqueous suspension polymerization of halogenated monomers. Dialkylperoxydicarbonates are particularly valued initiators because of theirhigh activity at the usual polymerization temperatures. However, theyexhibit the disadvantage of being unstable, so that their storage in thepure state exhibits very serious safety risks.

For the purpose of overcoming this disadvantage, provision has alreadybeen made to manufacture these dialkyl peroxydicarbonates in thepolymerization reactor (“in situ”). This “in situ” preparation processdoes not, however, make possible automation of the feeding of thepolymerization reactors with initiator. In addition, this process lacksreproducibility due to the lack of accuracy with respect to the amountsof initiator effectively employed in the polymerization. This processalso lacks productivity because it is necessary to precede eachpolymerization cycle by the “in situ” synthesis of the initiator.Furthermore, the by-products and residues from the synthesis of thedialkyl peroxydicarbonate are not removed.

Provision has also already been made to prepare the exact necessaryamount of dialkyl peroxydicarbonate outside the polymerization reactor(“ex situ”) and immediately before the polymerization. This preparationis carried out by reaction of an alkyl haloformate with a peroxidecompound in the presence of water and of a volatile water-immisciblesolvent preferably having a boiling temperature of less than 100° C. Thecombined reaction mixture in which the dialkyl peroxydicarbonate hasbeen prepared (aqueous phase and organic phase) is then introduced intothe polymerization reactor, which is subsequently charged for thepurpose of the polymerization (Belgian Patent 822,913 on behalf ofSolvay and Co.). The volatile solvent is preferably removed, in all orin part, before the polymerization by applying vacuum.

This process makes it possible to automate the feeding of thepolymerization reactors with initiator but still requires that the exactsufficient amount of initiator be produced immediately before thepolymerization. A delayed introduction of the initiator, which is anadvantageous technique, for example for improving the kinetics of thepolymerization, cannot consequently be carried out. Furthermore, thisprocess does not make it possible to have available a dialkylperoxydicarbonate solution which can be stored in complete safety andwhich can be used at any time. Furthermore, the water-soluble impuritiespresent in the aqueous phase after the preparation of the dialkylperoxydicarbonate are not removed before the introduction into thepolymerization reactor.

In order to overcome the disadvantages exhibited by the processes of theprior art, the object of the present invention is to provide an organicsolution of dialkyl peroxydicarbonate and more particularly of diethylperoxydicarbonate which is particularly suited to the preparation ofhalogenated polymers and more particularly of polymers comprisingfluorine (including vinylidene fluoride polymers) by aqueous suspensionpolymerization and an improved process for manufacturing it.

Another subject-matter of the invention is a simple and efficientprocess for the preparation of halogenated polymers and moreparticularly of polymers comprising fluorine (including vinylidenefluoride polymers) by aqueous suspension polymerization with theinvolvement of this organic solution.

Another subject-matter of the invention is the halogenated polymers andmore particularly the polymers comprising fluorine (including vinylidenefluoride polymers) thus obtained.

The present invention first of all relates to an improved process forthe preparation of an organic solution of dialkyl peroxydicarbonate,which solution is particularly suited for use in the aqueous suspensionpolymerization of halogenated monomers.

To this end, the invention relates to a process for the preparation ofan organic solution of dialkyl peroxydicarbonate according to which analkyl haloformate is reacted, in water, in appropriate amounts with aninorganic peroxide and the dialkyl peroxydicarbonate obtained isseparated by extraction by means of a water-insoluble organic solventchosen from conventional chain-regulating agents for halogenatedpolymers, in order to obtain a solution of dialkyl peroxydicarbonate inthis solvent.

The water-insoluble organic solvent is preferably chosen fromconventional chain-regulating agents for polymers comprising fluorineand, in a more than preferred way, it is chosen from conventionalchain-regulating agents for vinylidene fluoride polymers. In a veryparticularly preferred way, the water-insoluble organic solvent isdiethyl carbonate.

The dialkyl peroxydicarbonate is preferably diethyl peroxydicarbonate.

The alkyl haloformate is generally advantageously an alkylchloroformate. The inorganic peroxide is generally calcium or sodiumperoxide or alternatively hydrogen peroxide. In the latter case, it isadvisable, in addition, to introduce a base, such as calcium hydroxideor sodium hydroxide, into the aqueous reaction mixture. Preferably, theinorganic peroxide is hydrogen peroxide and sodium hydroxide is thenadded to the reaction mixture.

The amount of hydrogen peroxide is usually less than or equal to thestoichiometric amount. It is generally greater than or equal to astoichiometric shortage of 5% with respect to the amount of alkylhaloformate. The amount of sodium hydroxide is usually less than orequal to the stoichiometric amount. It is generally greater than orequal to a stoichiometric shortage of 5% with respect to the amount ofalkyl haloformate. The stoichiometric shortage does not necessarily haveto be the same for the hydrogen peroxide and the sodium hydroxide. Astoichiometric shortage of 3% for the sodium hydroxide and of 4% for thehydrogen peroxide with respect to the amount of alkyl haloformateusually gives good results.

The reaction between the alkyl haloformate, hydrogen peroxide and sodiumhydroxide is usually carried out with vigorous stirring. The temperatureof the reaction is generally maintained at a value between −5° C. and+15° C., preferably between 0° C. and +15° C. The total duration of thepreparation of the dialkyl peroxydicarbonate is regulated by theduration of the addition of the sodium hydroxide to the aqueous mixturecontaining the alkyl haloformate and the hydrogen peroxide, whichusually varies from a few tens of minutes to a few hours.

The separation by extraction of the dialkyl peroxydicarbonate obtainedis carried out in any known and appropriate way. Advantageously, theextraction solvent is added with vigorous stirring to the aqueousreaction mixture from the preparation of the dialkyl peroxydicarbonate,the phases are subsequently allowed to separate by settling, after thestirring has been halted, and the organic phase is separated from theaqueous phase, in order to collect a pure solution of the dialkylperoxydicarbonate in the extraction solvent.

The extraction solvent can be added to the aqueous reaction mixture atany point in the reaction for formation of the dialkylperoxydicarbonate, that is to say from the point ranging from theintroduction of the main reactants to after the synthesis of the dialkylperoxydicarbonate. Furthermore, the extraction solvent can be added allat once or in several steps.

The amount of solvent used for the extraction is not critical. It isobvious that it will depend in particular on the degree of solubility ofthe dialkyl peroxydicarbonate in the chosen solvent. This amount willadvantageously be such that the final concentration of dialkylperoxydicarbonate in the organic solution is comprised betweenapproximately 15 and approximately 40% by weight and more particularlybetween approximately 20 and between 35% by weight.

In the case where the relative density of the final aqueous phase afterformation of the dialkyl peroxydicarbonate is less than 1.05, it is thennecessary to density the aqueous phase by the addition of awater-soluble inorganic salt in an amount sufficient to increase therelative density of the aqueous reaction mixture and consequently tofacilitate the settling and the separation of the aqueous and organicphases.

The inorganic salt is then employed in an amount sufficient to bring therelative density of the aqueous reaction mixture to a value greater thanthe relative density of the organic solution produced in the secondphase. The relative density of the aqueous phase is preferably at leastequal to 1.05 and more particularly still to a value at least equal to1.06. Furthermore, it is advisable to adjust the amount of inorganicsalt so that it does not exceed the saturation concentration of salt inthe aqueous reaction mixture.

The nature of the salt employed in the stage of the preparation of thedialkyl peroxydicarbonate is not particularly critical. In principle,any inorganic salt which does not interfere with the reaction forformation of the dialkyl peroxydicarbonate and which does notprecipitate under the reaction conditions is suitable. Mention may bemade, as non-limiting examples of such salts, of, for example, alkalimetal and alkaline earth metal halides and in particular chlorides, inparticular sodium chloride, but also alkali metal and alkaline earthmetal sulphates, such as sodium sulphate, or alkali metal and alkalineearth metal nitrates, such as calcium nitrate.

The present invention also relates to an organic solution of dialkylperoxydicarbonate in a water-insoluble organic solvent which isparticularly suited for use in the aqueous suspension polymerization ofhalogenated monomers.

To this end, the invention relates to an organic solution of dialkylperoxydicarbonate in a water-insoluble organic solvent, according towhich this solvent is chosen from conventional chain-regulating agentsfor halogenated polymers.

The water-insoluble organic solvent is preferably chosen fromconventional chain-regulating agents for polymers comprising fluorineand, in a more than preferred way, it is chosen from conventionalchain-regulating agents for vinylidene fluoride polymers. In a veryparticularly preferred way, the water-insoluble organic solvent isdiethyl carbonate.

The dialkyl peroxydicarbonate is preferably diethyl peroxydicarbonate.

In addition, the present invention relates to an organic solution ofdialkyl peroxydicarbonate obtained by the preparation process formingthe subject-matter of the invention.

The concentration of dialkyl peroxydicarbonate in the organic solutionaccording to the invention is generally comprised between approximately15 and approximately 40% by weight. The concentration of dialkylperoxydicarbonate in the organic solution is preferably comprisedbetween approximately 20 and approximately 35% by weight.

The invention also relates to a process for the preparation ofhalogenated polymers by aqueous suspension polymerization of halogenatedmonomers with the involvement of dialkyl peroxydicarbonate, according towhich the dialkyl peroxydicarbonate is employed in the polymerization inthe form of an organic solution in a water-insoluble organic solventchosen from conventional chain-regulating agents for halogenatedpolymers.

The process for the preparation of halogenated polymers preferablyapplies to the preparation of polymers comprising fluorine and, in amore than preferred way, to the preparation of vinylidene fluoridepolymers.

The water-insoluble organic solvent is preferably chosen fromconventional chain-regulating agents for polymers comprising fluorineand, in a more than preferred way, it is chosen from conventionalchain-regulating agents for vinylidene fluoride polymers. In a veryparticularly preferred way, the water-insoluble organic solvent isdiethyl carbonate.

The dialkyl peroxydicarbonate is preferably diethyl peroxydicarbonate.

The concentration of dialkyl peroxydicarbonate in the solutions employedin the process for the preparation of halogenated polymers according tothe invention is generally comprised between approximately 15 andapproximately 40% by weight. Good results are obtained with solutions inwhich the concentration of dialkyl peroxydicarbonate is comprisedbetween approximately 20 and approximately 35% by weight.

The organic solution of dialkyl peroxydicarbonate according to theinvention is employed in amounts such that the dialkyl peroxydicarbonateis present in the polymerization mixture in usual amounts. Theconventional amounts of the dialkyl peroxydicarbonate are fromapproximately 0.05 to 3% by weight with respect to the monomer employedand preferably from approximately 0.05 to 2% by weight. The amounts ofthe chain-regulating agent employed in the polymerization areconventionally from approximately 0.5 to 5% by weight with respect tothe monomer employed. It may be necessary to add additional amounts ofthe chain-regulating agent in addition to those employed by theintroduction of the organic solution of dialkyl peroxydicarbonate in thetransfer agent.

The dialkyl peroxydicarbonates in organic solution are usuallyintroduced at the beginning of the polymerization. However, it isunderstood that the dialkyl peroxydicarbonates in organic solution canbe introduced, in all or in part, after the beginning of thepolymerization (delayed). The delayed use of a part of the dialkylperoxydicarbonate can be advantageous in improving the kinetics of thepolymerization.

Apart from the distinguishing feature of the use of a dialkylperoxydicarbonate in the form of an organic solution in awater-insoluble organic solvent according to the invention, the generalconditions of the polymerization according to the process of theinvention do not differ from those conventionally employed for thepreparation of halogenated polymers, more particularly polymerscomprising fluorine and in particular of vinylidene fluoride polymers,by aqueous suspension polymerization of halogenated monomers.

Aqueous suspension polymerization is understood to mean thepolymerization with the involvement of oil-soluble initiators, in thiscase in particular dialkyl peroxydicarbonates, and in the presence ofdispersing agent.

In the specific case of the preparation of vinylidene fluoride polymers,the dispersing agents are usually water-soluble cellulose derivatives,such as alkyl and alkylhydroxyalkylcelluloses. The amount of dispersingagent employed generally varies between 0.01 and 0.5% by weight withrespect to the monomer(s) employed. The best results are obtained whenuse is made of from 0.02 to 0.2% by weight thereof.

The polymerization temperature can be without distinction below or abovethe critical temperature of vinylidene fluoride (30.1° C.). When thetemperature is below 30.1° C., the polymerization is carried out in aconventional aqueous suspension of liquid vinylidene fluoride under apressure equal to the saturated vapour pressure of vinylidene fluoride.When the temperature is above 30.1° C., it is carried out in an aqueoussuspension of gaseous vinylidene fluoride which is advantageously underhigh pressure. It is thus possible to carry out the process according tothe invention at temperatures ranging from ambient temperature toapproximately 110° C. Nevertheless, it is preferable to carry out thepolymerization at a temperature above 30.1° C. According to a preferredembodiment of the process according to the invention, the polymerizationof vinylidene fluoride is carried out at a temperature of between 35 and100° C. and under initial pressures from approximately 55 to 200 bar. Ofcourse, it is possible to increase the productivity of the reactors bycarrying out, during polymerization, additional injections of monomer orof water or, during polymerization, raising the polymerizationtemperature.

The polymerization is generally carried out under batchwise conditions.It is generally carried out in vessel reactors provided with a blade,curved blade or turbine agitator.

At the end of polymerization, the vinylidene fluoride polymers obtainedaccording to the process of the invention are isolated in a conventionalway from their polymerization mixture, by a draining operation, followedby drying.

The invention also relates to halogenated polymers, characterized inthat they have at least one of the following properties which isimproved: a) thermal stability, b) purity.

Improved thermal stability is understood to mean that the yellowingindex YI, measured following the Boy test, and/or the yellowing index YI(10 min), measured following the Boy test (10 min), are/is improved.

Improved purity is understood to mean that the halogenated polymer ischaracterized by a reduced content of metal ions.

The halogenated polymers are preferably polymers comprising fluorineand, in a more than preferred way, vinylidene fluoride polymers.

The invention also relates to the halogenated polymers obtained by theprocess according to the invention.

The terms used in the present text are defined hereinbelow.

Organic solution of dialkyl peroxydicarbonate in a water-insolubleorganic solvent is understood to denote, for the purposes of the presentinvention, that the organic solution is composed essentially of thedialkyl peroxydicarbonate and of the water-insoluble organic solvent. Itis thus devoid of any aqueous phase originating from the reactionmixture in which the dialkyl peroxydicarbonate was prepared.

Dialkyl peroxydicarbonate is understood to denote, for the purposes ofthe present invention, peroxydicarbonates in which the alkyl radicalscomprise at least 2 carbon atoms and represent the ethyl, n-propyl,isopropyl, n-butyl, isobutyl, tert-butyl, n-octyl, 2-ethylhexyl,cyclohexyl, 4-tert-butylcyclohexyl, myristyl or cetyl radicals.Preference is given, among these, to diethyl and diisopropylperoxydicarbonates. A very particularly preferred dialkylperoxydicarbonate is diethyl peroxydicarbonate.

Water-insoluble organic solvent chosen from the chain-regulating agentsis understood to denote, for the purposes of the present invention, thechain-regulating agents which are liquid and insoluble in water understandard conditions, that is to say at ambient temperature and atatmospheric pressure.

Water-insoluble is understood to mean more particularly a solubility inwater at ambient temperature of less than 15% by weight. The solubilityin water of the chain-regulating agents acting as solvent for thedialkyl peroxydicarbonate in the process of the invention preferablydoes not exceed 5% by weight and, in a more than preferred way, it doesnot exceed 3% by weight.

Mention may be made, inter alia, among the chain-regulating agents whichcan be used for the purposes of the invention, of halogenatedderivatives, such as chloroform and trichlorofluoromethane, alkylacetates, in which the alkyl comprises from 2 to 6 carbon atoms, anddialkyl carbonates.

Among the chain-regulating agents which can be used for the invention,dialkyl carbonates are preferred.

The most effective dialkyl carbonates, to which preference isconsequently given, are those in which the alkyl groups comprise at mostfive carbon atoms. Mention may be made, as examples of such dialkylcarbonates, of dimethyl, diethyl, di(n-propyl), di(n-butyl),di(sec-butyl), diisobutyl, di(tert-butyl), di(n-pentyl), diisoamyl anddineopentyl carbonates.

A very particularly preferred dialkyl carbonate according to theinvention is diethyl carbonate (solubility in water at ambienttemperature: 1.9% by weight, relative density: 0.975).

Halogenated polymers is understood to denote, for the purposes of thepresent invention, both homopolymers and copolymers of halogenatedmonomers, in particular homopolymers of halogenated monomers, such asvinylidene fluoride, vinyl fluoride, trifluoroethylene,tetrafluoroethylene, chlorotrifluoroethylene, hexafluoropropylene, vinylchloride or vinylidene chloride, as well as copolymers of thesehalogenated monomers and copolymers of one of these halogenated monomerswith another monomer containing ethylenic unsaturation, such asethylene, acrylic or methacrylic monomers, or vinyl acetate.

Polymers comprising fluorine is understood to denote, for the purposesof the present invention, both homopolymers and copolymers of monomerscomprising fluorine, in particular homopolymers of vinylidene fluoride,of vinyl fluoride, of trifluoroethylene, of tetrafluoroethylene, ofchlorotrifluoroethylene or of hexafluoropropylene, as well as copolymersof these monomers comprising fluorine, such as, for example, thecopolymer of tetrafluoroethylene and of hexafluoropropylene, copolymersof vinylidene fluoride with another fluorinated monomer as defined aboveand copolymers of vinyl fluoride with another fluorinated monomer asdefined above. The copolymers of one of the abovementioned monomerscomprising fluorine with another monomer containing ethylenicunsaturation are also considered. The copolymer of tetrafluoroethyleneand of ethylene and the copolymer of trifluoroethylene and of ethyleneare examples thereof.

Vinylidene fluoride polymers is understood to denote, for the purposesof the present invention, both homopolymers of vinylidene fluoride andits copolymers with other monomers containing ethylenic unsaturationwhich are advantageously fluorinated. Mention may be made, as examplesof fluorinated comonomers which can be used, of vinyl fluoride,trifluoroethylene, chlorotrifluoroethylene, tetrafluoroethylene andhexafluoropropylene. The copolymers obtained preferably comprise atleast approximately 75% by weight of monomer units derived fromvinylidene fluoride. The said thermoplastic copolymers advantageouslyexhibit a melting temperature at least equal to 130° C. and preferablyat least equal to 150° C. and more particularly still to 165° C.

The process for the preparation of organic solutions of dialkylperoxydicarbonates according to the invention provides solutionsexhibiting numerous advantages. This is because, given that theimpurities which appear during the preparation are water-soluble and areremoved with the aqueous phase, the solutions obtained with a high yieldare very pure. The solutions of dialkyl peroxydicarbonates obtained bythe process according to the invention can be stored withoutdisadvantage for relatively long time periods (several months) withoutsignificant loss in activity. Storage is generally carried out at lowtemperature, preferably at a temperature below −10° C., preferably inthe region of −20° C. Relatively large amounts of the organic solutionof dialkyl peroxydicarbonate, sufficient for a large number ofpolymerization cycles, can thus be prepared and subsequently stored, inorder to be used as and when required. These solutions are ready-for-useand do not require the prior removal of the solvent, which has a role toplay during the polymerization. The same solution can be used to feedseveral polymerization reactors. Finally, the solutions obtained can betransported and do not result in problems of deposits in the pipes.

The process for the preparation of halogenated polymers according to theinvention has many advantages. In particular, it allows the feeding ofthe reactors to be automated. It results in an improvement in thereproducibility of the polymerization cycles. It also makes possible anincrease in the productivity. The polymerization process of theinvention also exhibits the advantage of involving a solution of thedialkyl peroxydicarbonate in a solvent which is itself a participant inthe polymerization. Furthermore, the use of the dialkylperoxydicarbonates in the form of an organic solution according to theinvention makes it possible to obtain resins exhibiting either a betterthermal stability or an increased purity or both, with respect to theresins obtained by preparing the dialkyl peroxydicarbonate “in situ”.

EXAMPLE 1 Preparation of a Solution of Dialkyl Peroxydicarbonate

1300 g of demineralized water are introduced into a 4 liter autoclaveplaced under vacuum beforehand. After having regulated the temperatureof the autoclave at +3° C., 111.7 g of 70% aqueous hydrogen peroxidesolution and 600 g of ethyl chloroformate, cooled beforehand to between0 and 5° C., are subsequently successively introduced into the aqueoussolution, which is stirred by means of a curved blade agitator (speed of400 revolutions/minute), while the autoclave is still under vacuum. Thepiping conveying the ethyl chloroformate is then rinsed with 500 g ofdemineralized water and the speed of stirring is increased to 750revolutions/minute. The autoclave is subsequently brought to atmosphericpressure before slowly introducing 780 ml of a 6N sodium hydroxidesolution over 1 hour. The temperature is regulated at +3° C.±2° C.throughout the introduction of the sodium hydroxide. After the end ofthe introduction of the sodium hydroxide, the reaction mixture is keptstirring for 10 minutes. The temperature of the autoclave issubsequently indexed at +1° C. and, under a speed of stirring reduced to200 revolutions/minute, 1300 g of diethyl carbonate, cooled beforehandto between 0 and 5° C., are introduced. After having kept the reactionmixture stirring at 750 revolutions/minute for 10 minutes, the stirringis then halted and separation by settling is allowed to take place for30 minutes. The autoclave is subsequently brought back to atmosphericpressure and the organic and aqueous phases are withdrawn. 1631 g oforganic solution are thus collected, which solution has a diethylperoxydicarbonate assay of 245 g/kg, i.e. a yield of 85% with respect tothe hydrogen peroxide. The diethyl peroxydicarbonate solution thusproduced is stored under cold conditions (approximately −20° C.) for thepurpose of its subsequent use.

EXAMPLE 2 Preparation of a Halogenated Polymer

19 kg of demineralized water and 270 g of a 15 g/kg aqueousethylhydroxypropylcellulose solution are successively introduced into a30.8 liter reactor which is vigorously stirred and equipped with ajacket. The greater part of the oxygen present in the reactor is removedby placing under a vacuum of 40 mbar (at 15° C.) three times with, afterthe first two of these operations, repressurizing with 1 bar ofnitrogen. 82 g of a 24.5% by weight solution of diethylperoxydicarbonate in diethyl carbonate are then introduced under vacuum.An additional 131 g of pure diethyl carbonate are added. A single chargeof 8 kg of vinylidene fluoride is subsequently introduced and then thereactor is gradually heated until a first stationary temperature phaseof 42° C. is reached, for a duration of approximately 1 hour 30 minutes.The temperature is subsequently brought to 61° C. and is maintainedthere for approximately 1 hour 45 minutes. At the end of polymerization,the aqueous suspension is degassed (the pressure being lowered toatmospheric pressure). The polymer collected is washed and dried toconstant weight. The total duration of the polymerization is 3 hours 50minutes. The degree of conversion measured is 94%.

EXAMPLE 3 Properties of the Halogenated Polymer Obtained in Example 2

The MFI (Melt Flow Index) of the polymer obtained, measured according toISO Standard 1133 at a temperature of 230° C. and under a weight of 2.16kg, is 12 g/10 minutes. The yellowing index YI, measured following theBoy test, and the yellowing index YI (10 min), measured following theBoy test, 10 min. are respectively 11 and 20. The Boy test and themeasurement method for determining the yellowing index YI are describedhereinbelow. The content of metal ions and in particular the content ofsodium ions is less than 1 mg/kg of poly(vinylidene fluoride). Themethod of determination of the content of metal ions is describedhereinbelow.

Description of the Thermal Stability Test (Boy Test)

The poly(vinylidene fluoride) is subjected to a thermal and mechanicalstress during injection of a test specimen with an injection mouldingmachine. The article to be injected, with dimensions of 60×40×4 mm, isinjected via a shell (31.5×0.8 mm). The mould is connected to a thermalregulation system. The temperature of the mould is regulated at 60±2° C.The injection moulding machine used is a Boy 15 S injection mouldingmachine, which is characterized by a closing force of 22 tonnes, acylinder with a diameter of 22 mm, an open nozzle with a diameter of 2.1mm as injection nose, a non-return valve composed of 3 V-shapedchannels, a screw characterized by an L/D ratio of 20, 3 zones(12D/4D/4D) and a compression ratio of 1.9. Two independent heatingresistance elements are placed on the barrel (zones 1 and 2) and tworesistance elements coupled to the zone of the non-return valve and theinjection nose (zone 3). The temperatures of these 3 heating zones are,in order, 190° C., 220° C. and 265° C. The residence time in the barrelis of the order of 5 minutes. The thermal effect can be increased bymaintaining the hot molten polymer in the barrel of the machine, theinjection cycle being interrupted for 10 minutes (Boy test, 10 min).

Description of the Measurement Method for the Determination of theYellowing Index YI

The yellowing index YI of the samples is determined according to ASTMStandard D-1925 by means of a Hunterlab “Ultrascan” spectrocolorimeterregularly calibrated by virtue of 3 Hunterlab colorimetric calibratingstandards (white, black grey). The measurement reference system is theCIELAB 1976, the illuminant is D65 from the CIE, the spectral bandanalysed is from 400 to 700 nm, the spectral measurement interval is 5nm, the diameter of the range examined is 0.95 cm, the standard observeris at 10° and observation is carried out under an angle of 8°.

Description of the Method for the Determination of the Content of MetalIons

The content of metal ions, in particular the content of sodium ions,expressed as mg/kg of poly(vinylidene fluoride), is measured by atomicabsorption spectrometry in an air-acetylene flame (F-AAS) at awavelength of 589.0 nm. Before analysis, the sample is first subjectedto mineralization by sulphuric acid in a platinum dish, followed bycalcination at 580° C., the ash being taken up in 5 mol/literhydrochloric acid and made up to volume in the presence of caesium(spectral gauge).

EXAMPLE 4 COMPARATIVE Properties of a Halogenated Polymer Obtained withthe Involvement of a Dialkyl Peroxydicarbonate Prepared “in situ”

By way of comparison, the polymerization of vinylidene fluoride wasrepeated under the same conditions as in Example 2, except that theappropriate amount of diethyl peroxydicarbonate is first synthesized “insitu” in the polymerization reactor. Thus, 19 kg of demineralized water,9 g of sodium hydroxide, 38.3 g of 10% aqueous hydrogen peroxidesolution and 270 g of a 15 g/kg aqueous ethylhydroxypropylcellulosesolution are successively introduced into a 30.8 liter reactor which isvigorously stirred and equipped with a jacket. After deaerating theautoclave by placing under vacuum and repressurizing with nitrogen, 24.5g of ethyl chloroformate and 190 g of diethyl carbonate, aschain-regulating agent, are added. A single charge of 8 kg of vinylidenefluoride is subsequently introduced and then the reactor is graduallyheated until a first stationary temperature phase of 42° C. is reached,for a duration of approximately 2 hours. The temperature is subsequentlybrought to 61° C. and is maintained there for approximately 2 hour. Atthe end of polymerization, the aqueous suspension is degassed (thepressure being lowered to atmospheric pressure). The polymer collectedis washed and dried to constant weight. The total duration of thepolymerization is 4 hours 40 minutes. The degree of conversion measuredis 94%.

The MFI (Melt Flow Index) of the polymer obtained in Example 4, measuredaccording to ISO Standard 1133 at a temperature of 230° C. and under aweight of 2.16 kg, is 10 g/10 minutes. The yellowing index YI, measuredfollowing the Boy test, and the yellowing index YI (10 min), measuredfollowing the Boy test, 10 min, are 13 and 29 respectively. The contentof metal ions and in particular the content of sodium ions is 2 mg/kg ofpoly(vinylidene fluoride).

From the comparison of the results, it is apparent that the use ofdiethyl peroxydicarbonate in solution in diethyl carbonate (according tothe invention) in the polymerization of vinylidene fluoride gives riseto the preparation of vinylidene fluoride polymers which aresignificantly more stable thermally and which have a higher ionic purity(Examples 2 and 3) than those obtained with the involvement of diethylperoxydicarbonate prepared “in situ” (Comparative Example 4).

What is claimed is:
 1. A process for the preparation of halogenatedpolymers by aqueous suspension polymerization of halogenated monomerswith the involvement of dialkyl peroxydicarbonate, characterized in thatthe dialkyl peroxydicarbonate is employed in the polymerization in theform of an organic solution in a water-insoluble organic solvent chosenfrom dialkyl carbonates.
 2. The process according to claim 1,characterized in that the halogenated polymers are polymers comprisingfluorine.
 3. The process according to claim 1, characterized in that thehalogenated polymers are vinylidene fluoride polymers.
 4. The processaccording to claim 1, characterized in the water-insoluble organicsolvent is diethyl carbonate.
 5. The process according to claim 1,characterized in that the dialkyl peroxydicarbonate is diethylperoxydicarbonate.
 6. A process for the preparation of halogenatedpolymers by aqueous suspension polymerization of halogenated monomerswith the involvement of diethyl peroxydicarbonate, characterized in thatthe diethyl peroxydicarbonate is employed in the polymerization in theform of an organic solution in a water-insoluble organic solvent chosenfrom conventional chain-regulating agents for halogenated polymers. 7.The process according to claim 6, characterized in that the halogenatedpolymers are polymers comprising fluorine.
 8. The process according toclaim 6, characterized in that the halogenated polymers are vinylidenefluoride polymers.
 9. The process according to claim 6, characterized inthat the water-insoluble organic solvent is chosen from conventionalchain-regulating agents for polymers comprising fluorine.
 10. Theprocess according to claim 6, characterized in that the water-insolubleorganic solvent is chosen from conventional chain-regulating agents forvinylidene fluoride polymers.